Error messages in 2008 Books Online

I just learned from Gail E. at MS that the system error messages are all documented in the SQL Server 2008 Books Online.

For instance, search for 823. See the hits? Now, for instance, select the "Errors 1 - 999" page. Press the "Sync with Table of Contents" button. Look to the left, in the "Contents" section. You now see that the errors messages are divided into sections (1-999, 1000-1999, ...). You also see where in BOL to find these sections: